TWO LANE HIGHWAYS

CAPACITY

Capacity = 1,700 pc/h for each direction, and 3,200 for both directions combined

The capacity of a two-lane highway is 1,700 pc/h (passenger cars per hour) for each direction of travel. The capacity is nearly independent of the directional distribution of traffic on the facility, except that for extended lengths of two-lane highway, the capacity will not exceed 3,200 pc/h for both directions of travel combined. For short lengths of two-lane highway—such as tunnels or bridges—a capacity of 3,200 to 3,400 pc/h for both directions of travel combined may be attained but cannot be expected for an extended length.

If the volume of traffic is greater than 1700 pc/h in one direction, or greater than 3200 pc/h in both directions, then we are over capacity (LOS F) and no further analysis is needed to determine how the facility is performing.

LEVEL OF SERVICE

The performance of a Class I two-lane highway in non-mountainous terrain and no traffic signals is measured in relation to Average Travel Speed (ATS) and Percent Time Spent Following (PTSF). ATS and PTSF are the Performance Criteria for 2-Lane Highways.

DETERMINING AVERAGE TRAVEL SPEED

The average travel speed (ATS) is estimated from the Free Flow Speed (FFS), the demand flow rate (Vp), and an adjustment factor for the percentage of no-passing zones (f_np):

Estimating FFS

The FFS can be estimated indirectly if field data are not available using the following equation, where BFFS = Base Free Flow Speed, f_LS = an adjustment factor for lane width and shoulder width, and f_A = an adjustment factor for number of access points per mile.

The design speed and posted speed limit of the facility may be considered in determining the BFFS. The speed limit plus 5 mph or the design speed plus 5 mph are frequently used.

The first adjustment (f_LS) relates to the effects of lane and shoulder widths on Free Flow Speed. Base conditions for a two-lane highway require 12-ft lane widths and 6-ft shoulder widths.

The second adjustment (f_A) relates to the effect on Free Flow Speed of traffic entering and exiting intersecting streets and driveways. The data indicates that each access point per mile decreases the estimated FFS by about 0.25 mi/h. The access point density is found by dividing the total number of intersections and driveways on both sides of the roadway segment by the length of the segment in miles. An intersecting street or driveway should only be included if it influences traffic flow; access points with little activity should not be included.

Determining Demand Flow Rate (Vp)

Three adjustments must be made to hourly demand volumes (V), whether based on traffic counts or estimates, to arrive at the equivalent passenger-car flow rate (V_P) used in LOS analysis. These adjustments are the PHF, the grade adjustment factor (f_G), and the heavy vehicle adjustment factor (f_HV). These adjustments are applied using the following equation:

Grade Adjustment (f_G)

The grade adjustment factor (f_G) accounts for the effect of the terrain on travel speeds and percent time-spent-following, even if no heavy vehicles are present.

Adjustment for Heavy Vehicles (f_HV)

The presence of heavy vehicles in the traffic stream decreases the FFS (Free Flow Speed) because, at base conditions, the traffic stream is assumed to consist only of passenger cars. Therefore, traffic volumes must be adjusted to an equivalent flow rate expressed in passenger cars per hour. This adjustment is accomplished by using the factor f_HV.

Adjustment for the presence of heavy vehicles in the traffic stream applies to two types of vehicles: trucks and RVs. Buses should not be treated as a separate type of heavy vehicle but should be included with trucks. The heavy-vehicle adjustment factor requires two steps. First, the passenger-car equivalency factors for trucks (E_T) and RVs (E_R) for the prevailing operating conditions must be found. Then, using these values, an adjustment factor must be computed to correct for all heavy vehicles in the traffic stream.

Once values for E_T and E_R have been determined, the adjustment factor for heavy vehicles (f_HV) is computed using the following equation where P_T is the percentage of trucks in the traffic stream (expressed as a decimal) and P_R is the percentage of RV’s in the traffic stream (also expressed as a decimal):

Iterative Computations for Vp

If the computed value of Vp is less than the upper limit of the selected flow-rate range for which f_G, E_T, and E_R were determined, then the computed value of Vp should be used. If the Vp is higher than the upper limit of the selected flow-rate range, repeat the process for successively higher ranges until an acceptable value of Vp is found. Because the highest range includes all flow rates greater than1200 pc/h in both directions of travel combined, it can be used if a computed value exceeds the upper limit of both lower flow-rate ranges.

Estimating f_np

DETERMINING PERCENT TIME-SPENT-FOLLOWING

The percent time-spent-following is estimated from the demand flow rate, the directional distribution of traffic, and the percentage of no-passing zones.

Estimating BPTSF

Base Percent Time Spent Following (BPTSF) is calculated using the formula:

Vp is calculated exactly as before except that the exhibits pertaining to percent-time-following are used instead of the exhibits pertaining to average travel speed to determine f_G and f_HV

Estimating f_d/np

Finally, an adjustment representing the combined effect of the directional distribution of traffic and the percentage of no-passing zones is also made.